Hitherto, as a magnetic recording medium, in addition to a coating type magnetic recording medium formed by coating a dispersion of a ferromagnetic powder in an organic high molecular compound binder on a non-magnetic support, a so-called metal thin layer type magnetic recording medium has been used having a ferromagnetic metal thin layer formed by a vapor deposition method (such as a vacuum vapor deposition method, a sputtering method, an ion plating method, etc.) or a plating method (such as an electroplating method, an electroless plating method, etc.), as the magnetic layer for high density recording. This so-called metal thin layer type magnetic recording medium does not use a binder.
As a method of forming a magnetic layer, having the desired coercive force and squareness ratio, for a magnetic recording medium by a vacuum vapor deposition (which is a typical thin film (or layer) forming method), an oblique-incident vapor deposition method is known as disclosed in U.S. Pat. Nos. 3,342,632 and 3,342,633.
In regard to the magnetic recording medium having a ferromagnetic metal thin film as the magnetic layer, desired properties cannot be achieved with respect to weather resistance, abrasion resistance, and running property.
That is, a magnetic layer composed of a ferromagnetic metal thin film has a tendency to corrode, although the magnetic layer has excellent recording and reproducing characteristics. Also, a magnetic recording medium is placed in relative high speed movement with a magnetic head during recording, reproducing, and erasing of magnetic signals. For a metal thin layer type magnetic recording medium having a very smooth surface, it is difficult to smoothly and stably run the magnetic recording medium, and to prevent abrasion and/or the destruction with a magnetic head. Therefore, it is very important for the practical use of a metal thin layer type magnetic recording medium to overcome these difficulties.
In view of the foregoing, a lubricant layer or a protective layer provided on the surface of the metal thin layer type magnetic recording medium (for improving the weather resistance and the running property thereof) has been considered and investigated.
Surface protective materials for the metal thin layer type magnetic recording medium include oxides such as silicon oxide, etc., formed in a vapor phase or a liquid phase; organic polymers formed by a sputtering, a plasma polymerization, etc.; diamond-like carbon; graphite-like carbon; thermoplastic resins; thermosetting resins; etc.
A layer formed by coating a solution of a lubricating material such as fatty acids, metal salts of fatty acids, fatty acid esters, alkylphosphoric acid esters, perfluoropolyether components, polar group-modified perfluoropolyether series compounds, etc., and a layer formed by a Langmuir Blodgett method as described, e.g., in JP-A-60-69824 and JP-A-60-85427 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application") are known as the foregoing lubricant layer.
Also, it has been attempted to coat the surface of the metal thin layer type magnetic recording medium with an organic silicone compound(s). For example, JP-A-62-89228 discloses a technique for applying a siloxane polymer or a ladder type silicone compound onto the surface of the magnetic layer, and JP-A-54-45103 and JP-A-59-77630 both disclose a technique for treating or coating the surface of the magnetic layer with a coupling agent such as an alkoxysilane, etc.
However, in particular, when the protective layer is thin, the metal thin layer type magnetic recording medium has the disadvantages that the electromagnetic characteristics are deteriorated when stored at a high temperature and in high humidity. Further, the running property such as high speed abrasion resistance and abrasion resistance to materials having smooth surfaces under a severe condition (such as low humidity, high humidity, etc.) are insufficient.
These disadvantages are caused by the foregoing silicone coatings because the molecular weight of the silicone existing at the surface of the magnetic layer cannot be significantly increased.
That is, the foregoing disadvantages are caused due to the requirement that the layer of the silicone compound be formed on the magnetic layer by dissolving the silicone compound in an organic solvent and coating the solution on the magnetic layer.